Incorrect query after using DateTime.MinValue in the DateRange filter

[IvanSorokin]

NEST/Elasticsearch.Net version: 6.4.0

Elasticsearch version: 6.5.1

Description of the problem including expected versus actual behavior:
I am using filter for counting documents like this

private static Func<QueryContainerDescriptor<ISearchableNotification>, QueryContainer> IndexedDateFromFilter(SearchQuery query)
{
    if (query.IndexedDateFrom.HasValue)
        return filter => filter.DateRange(rf => rf.Field(nm => nm.IndexedDateTime)
                                                  .GreaterThanOrEquals(query.IndexedDateFrom));
     return null;
}

IndexedDateFrom's type is [DateTime?]. When IndexedDateFrom equals DateTime.MinValue everything crashes with exception

org.elasticsearch.ElasticsearchParseException: failed to parse date field [] with format [strict_date_optional_time||epoch_millis]

because NEST translates C# code into this (part of the query I got from DebugInformation)

{
                "range": {
                  "indexedDateTime": {
                    "gte": ""
                  }
                }
}

If I use DateTime.MinValue.AddDays(1) instead of DateTime.MinValue everything works fine. It looks like a bug.

[Mpdreamz]

Opened #3546 to fix this as best as we can in 6.5.

Need to open another PR to make DateMath anchor a DateTime? on master (7.0).